Primary magmas and mantle temperatures through time

Chemical composition of mafic magmas is a critical indicator of physicochemical conditions, such as pressure, temperature, and fluid availability, accompanying melt production in the mantle and its evolution in the continental or oceanic lithosphere. Recovering this information has fundamental implications in constraining the thermal state of the mantle and the physics of mantle convection throughout the Earth's history. Here a statistical approach is applied to a geochemical database of about 22,000 samples from the mafic magma record. Potential temperatures (T-ps) of the mantle derived from this database, assuming melting by adiabatic decompression and a Ti-dependent (Fe2O3/TiO2 - 0.5) or constant redox condition (Fe2+/Sigma Fe= 0.9 or 0.8) in the magmatic source, are thought to be representative of different thermal horizons'' (or thermal heterogeneities) in the ambient mantle, ranging in depth from a shallow sublithospheric mantle (T-p minima) to a lower thermal boundary layer (T-p maxima). The difference of temperature (Delta T-p) observed between T-p maxima and minima did not change significantly with time (similar to 170 degrees C). Conversely, a progressive but limited cooling of similar to 150 degrees C is proposed since similar to 2.5 Gyr for the Earth's ambient mantle, which falls in the lower limit proposed by Herzberg et al. [ 2010] (similar to 15-250 degrees C hotter than today). Cooling of the ambient mantle after 2.5 Ga is preceded by a high-temperature plateau evolution and a transition from dominant plumes to a plate tectonics geodynamic regime, suggesting that subductions stabilized temperatures in the Archaean mantle that was in warming mode at that time.